1. Field of the Invention
The present invention relates generally to solvents for solubilization and processing of organic polymers to useful forms as films, coatings, or fibers and, more particularly, to solvent systems containing Lewis acids and aprotic organic liquids for producing isotropic or anisotropic solutions of rigid chain or ladder polymers.
2. Related Art
Numerous rigid chain or ladder polymers have been synthesized in the past few decades and shown to exhibit exceptional high temperature stability, very high mechanical strength, resistance to solvents, and generally excellent environmental stability. Polymeric materials with these properties are of technological interest for diverse applications, including as structural materials in engineering structures and aerospace vehicles, as dielectric materials in electronics, as engine components, and as heat-resistant coatings.
Rigid chain polymers are exemplified by poly(p-phenylene-2,6-benzoxazole) (PBO), of the structure ##STR1## poly(p-phenylene-2,6-benzothiazole) (PBT), of the structure ##STR2## poly(p-phenylene-2,6-benzimidazole) (PBI), of the structure ##STR3## and the so-called ladder structures, for example, by benzimidazobenzo-phenanthroline-type ladder polymer (BBL) and derivatives, five of whose structures are also shown below. ##STR4## Other examples of ladder polymers include the polyhydroquinoxaline ladder structure, ##STR5## whose oligomeric model compound 5,12-dihydro-5,7, 12,14-tetraazapentacene (DHTAP) has the structure ##STR6##
The semi-ladder polybenzimidazobenzophenan-throline (BBB) of structure, ##STR7## and its oligomeric model compound cis-BB of the structure, ##STR8## also exemplify the rigid chain and thermally stable polymers in structural and physical properties. Other notable polymers in this regard include the non-fused polybenzimidazole (X=N-H) and derivatives with the structure, ##STR9## and the aromatic polyimides of the structure such as poly [N,N'-bis(phenoxyphenyl)pyrromellitimide) or Kapton (Trademark of the duPont Co).
More recently, these rigid chain or ladder polymers, such as exemplified in structures I to XIV, have shown useful electronic and optical properties in addition to their excellent thermal stability and mechanical properties and thus are of further technological interest as novel materials for electronic, electro-optic, and optical applications.
However, these rigid chain and ladder polymers and even those modified with pendant groups, have generally been insoluble in aprotic organic solvents from which their solutions could be processed by standard polymer film, coating or fiber processing techniques. Furthermore, their melting points are generally above their decomposition temperatures which are well in excess of 400.degree.-600.degree. C. The general insolubility of these types of polymers in organic solvents can be attributed to two main factors inherent in their molecular architecture: (a) intramolecular or conformational effects, particularly the rigidity of the polymer chains; and (b) intermolecular effects, especially strong interchain interactions.
Heretofore, strong and corrosive concentrated acids, such as methanesulfonic acid (MSA), triflic acid (CF.sub.3 SO.sub.3 H), and sulfuric acid, are solvents which have been used to characterize their properties and process them into films or fibers. The observed solubility of these polymers in strong protic acids is generally thought to originate from protonation of the polymer chains to form polyelectrolytes, such as shown below for BBL, ##STR10## with consequent significant reduction of the intermolecular attractions and the rigidity of the chain. The major disadvantages of using these strong acid solutions for large-scale production of films and fibers of these rigid chain and ladder polymers include the highly corrosive and toxic nature of the solvents; in addition, the acids are not volatile and are generally difficult to remove from the polymer films or fibers.
In the related art, processing of rigid chain para-linked aromatic polyamides such as poly(1,4-benzamide) (PBA), ##STR11## and poly(1,4-phenyleneterephthalamide) (PPTA) or Kevlar (Trademark of the E.I. duPont Co.), ##STR12## to fibers and films is achieved by their successful solubilization in a binary solvent system consisting of LiCl or CaCl.sub.2 dissolved in amide solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc and N-methylpyrrolidone (NMP).
Typically, PBA, PPTA or related rod-like aromatic polyamides can be dissolved in the aprotic organic solvents (DMF, DMAC, NMP) containing 3-15% wt LiCl or CaCl.sub.2. At sufficiently high PBA or PPTA concentrations in LiCl/amide or CaCl.sub.2 /amide solvents, liquid-crystalline solutions can be obtained. Such liquid-crystalline polymer solutions are used to prepare high-strength, high-modulus oriented fibers as disclosed by S. L Kwolek, et al (U.S. Pat. Nos. 3,600,350 and 3,819,587, and Macromolecules 10, 1390-1396, 1977). However, these salt/amide solvent systems, such as LiCl/DMF, LiCl/DMAc, LiCl/NMP, etc. do not solubilize the class of rigid chain thermally stable polymers exemplified by the polymer structures I to XIV.
Some researchers have reported successful solubilization of certain rigid chain, conjugated, or ladder polymers in pure Lewis acids in the molten or liquid state. One of the present inventors, S. A. Jenekhe, and his co-workers have disclosed the successful solubilization of polycarbazoles of the structure, ##STR13## in molten iodine (m.p. =106.degree. C.), resulting in conducting polymer solutions from which doped conducting films of polycarbazoles can be obtained (Jenekhe et al, U.S. Pat. No. 4,568,482 of common assignment with the present invention) and Molecular Crystals and Liquid Crystals 105, 175, 1984). The solubilization of poly(p-phenylene sulfide) (PPS) of the structure, ##STR14## where X=S, in liquid AsF.sub.3 /AsF.sub.5 to produce conducting solutions from which conducting films of PPS can be obtained has been reported by Frommer (J.E. Frommer, Accounts of Chemical Research 19, 2-9, 1986).
The solubilization of a rigid chain polymer, polybisbenzimidazobenzophenathroline-dione (BBB), of the structure XII, in molten antimony trichloride (SbCl.sub.3) (m.p. =73.C) has been reported by Berry et al (G. C. Berry and S. M. Liwak, J. Polym. Sci.: Polym. Phys. Ed. 14, 1717, 1976). In studies by the present inventors, some of the rigid chain and ladder polymers of structures I-XIV, and related polymers, have been found to dissolve in some liquid or molten pure Lewis acids such as SbCl.sub.3 and AsF.sub.3 but do not dissolve in others such as SbCl.sub.5, and SbF.sub.5.
Nevertheless, these pure liquid or molten Lewis acids are not practical solvents for potential large-scale processing of the rigid chain thermally stable polymers to films, coatings, or fibers, for obvious technical, environmental, and economic reasons. Thus, the problems of solubilization of rigid chain thermally stable polymers in suitable organic solvents, and consequently, the lack of methods for their ready processing to films, coatings, fibers, and other forms, remain a major obstacle to the commercial uses of these polymers which otherwise have many desired and interesting electronic, optical, thermal stability, and mechanical properties.